This invention relates to the production of low density, cellulosic products such as fibrous thermal insulation, and especially to an improved method and apparatus for producing such products. More particularly, the invention relates to a novel method and apparatus that utilize the energy generated by producing a high velocity flow of air with shreds of feedstock entrained therein, combined with mechanical action to fiberize the material with minimal damage to the fibers themselves.
Typically, dry process comminuting of organic materials for use as thermal insulation, absorbent pads, filters, and the like is achieved by using conventional hammer mills.
Hammer mills for performing the comminuting operation are shown in the following U.S. Pat. Nos.:
______________________________________ 1,777,905 2,494,107 1,934,180 2,505,023 2,045,582 3,143,303 2,082,419 3,429,349. 2,098,480 ______________________________________
It has been found that the use of hammer mills cannot produce a fiberized mass that will optimize the physical properties of low mass densities, high thermal resistance to heat flow, high moisture absorbence, and an acceptable aesthetic appearance.
For example, cellulose thermal insulation produced in conventional hammer mills results in products containing less than 50 percent of the mass at optimum fiber size needed to provide a low weight per cubic foot and high resistance to heat flow (R value). Typically, these products contain large (0.250 to 0.500 inch diameter) pieces of unfibered material and a large percentage of fines or dust.
In a given volume of such insulation, the following particle sizes may be observed:
______________________________________ Coarse pieces 20 to 40% Optimum fiber size less than 50% Fines or dust 10 to 30% ______________________________________
Hammer mill design, as is apparent from the above-listed patents, utilizes hammers or beaters that are pivotally mounted on a series of disks that rotate within a partial cylindrical sizing screen. The feedstock is typically fed into the mill via an airstream flowing perpendicular to the rotating hammers. The entire mass of feedstock is then drawn down into a wedge-shaped space and onto the beginning of the sizing screen comprising a major pinch point and then forced through and over a typical semicylindrical screen.
Due to the extraordinary pressure exerted on the screen at the entry pinch point, heavy gauge 3/16 to 1/4 inch thick, perforated metal screens are needed to prevent breakage from fatigue. The heavy gauge further limits the perforated open area to 30 or 40% and restricts the possible use of smaller perforations.
As a result of the input feed method, the swing hammers will retract as the feedstock is worked through the screen, thereby reducing the air flow due to a relatively thick mat of material, blinding the screen, and increasing the feed residence time within the machine, resulting in fines and dust. This deficiency is often mitigated by using screens with larger perforations. This results in large unfibered pieces remaining in the product.
Another deficiency is that the hammers are supported between disks, which, in turn, prevent complete utilization of the comminuting screen surface, adding to the blinding of the perforations. As most of the systems are set up to be air-swept, blinding of perforations can have a major negative effect by retarding air flow and increasing energy consumption and product degradation.
Other types of comminuting or disintegrating apparatus have been developed for producing fibrous, cellulosic product, such as thermal insulation, and typical units are disclosed in the following U.S. Pat. Nos.:
______________________________________ 1,749,954 3,986,676 3,255,793 3,987,968 ______________________________________
While these devices are capable of producing product without the pulverizing effect caused by hammer mills, they do not reduce many of the disadvantages outlined above.
The method and apparatus of the present invention, however, resolve many of the problems listed above and provide other features and advantages heretofore not obtainable.